Tempertature Sensor for a Cooking Device, Cooking Device With Electronic Control and Method for Temperature Recording

ABSTRACT

A temperature sensor on a cooking device, for non-contact recording of a temperature of a cooking utensil includes at least one infra red-sensor, with at least two differing sensitivity ranges for recording at least two different wavelengths emitted by the cooking utensil. The infra-red sensor can be a multi-channel pyrometer. In accordance with a method for non-contact recording of a temperature of a cooking utensil, at least two different wavelength ranges emitted by the cooking utensil are recorded by an infra-red sensor comprising a multi-channel pyrometer.

The present invention relates to a temperature sensor for a cookingdevice according to the preamble of claim 1 for non-contact recording ofa temperature of a cooking utensil. The invention further relates to acooking device having the features of claim 3 and a method fornon-contact recording of a temperature of a cooking utensil having thefeatures of claim 5.

More simply equipped cooking devices do not have circuits for regulatinga cooking temperature. The cooking temperature can only be coarselypre-selected by means of adjusting devices. During the cooking processthe temperature at the hot plate then fluctuates to a greater or lesserextent. In better equipped cooking devices, a control circuit can beprovided to keep a pre-selected temperature constant. A necessary inputquantity is a measured temperature at the cooking surface and/or at thecooking utensil.

Known non-contact temperature sensors are used to record thetemperatures of cooking utensils located on the cooking surfaces incooking devices. Known temperature sensors comprise an infrared sensorwhich records the temperature of the outer wall of the cooking utensilin a wavelength range of about 6 μm to about 14 μm by means of acontinuous light pyrometer. The electrical signal supplied by theinfrared sensor for the electronic control depends directly on theemittance of the pot side being considered. In order to achieve a signalwhich is as reliable as possible and free from interference, thisemittance must be known exactly in the wavelength range beingconsidered. This emittance in the wavelength range being considered isalso designated as band emittance.

However, the emittances of different cooking utensils differ as a resultof the different materials in some cases. The emittance of stainlesssteel pots differs substantially from the emittance of enamelled pots.Using pots having an unsuitable enamel coating causes a shift of thefood temperatures achieved in the pot. This is caused by fluctuations inthe band emittance of the enamel coating. Contaminants on the coatingcan present problems when making an exact determination.

A method for non-contact radiation measurement of the object temperatureindependent of the emittance of the object being considered is knownfrom EP 01 143 282 A. A method for recording a temperature distributionof a cooking utensil is furthermore known from EP 1 302 759 A.

An object of the present invention is to provide a non-contact sensorfor recording a cooking utensil temperature which delivers a temperaturesignal which is as reliable as possible.

This object is achieved with the subject matter of the independentclaim. A temperature sensor of a cooking device according to theinvention for non-contact recording of a temperature of a cookingutensil comprises at least one infrared sensor for recording at leasttwo different wavelength ranges which are emitted by the cookingutensil. In particular, a multi-channel pyrometer disposed on a hotplate in the immediate vicinity of the cooking utensil can be consideredas an infrared sensor of this type. It is hereby possible to obtain areliable prediction of the temperature inside the cooking utensil evenfor different materials of the cooking utensil.

An infrared sensor of this type records thermal radiation emitted by theobject being considered in a defined wavelength range λ (where λ₁≦λ≦λ₂)and converts this into an electrical signal. The electrical signalS₁₂(T) delivered by the infrared sensor uniquely designates thetemperature of the surface being considered. The band emittance ε₁₂ ofthe surface being considered is contained as a multiplicative factor inthe recorded signal S₁₂(T). The signal S₁₂(T) can be deduced accordingto the following equation (1)S ₁₂(T)=S _(P,12)(T)*ε₁₂ *C ₁₂,where the term S_(P,12)(T) is the theoretical signal of a Planck emitterin the wavelength range (λ₁≦λ≦λ₂) being considered. The factor C₁₂ is acorrection factor from the optics of the sensor or from the size of thesurface being considered for the measurement.

Assuming that in two separate wavelength ranges λ₁≦λ≦λ₂ and λ₃≦λ≦λ₄(where λ₂<λ₃), the two respective band transmittances ε₁₂ and ε₃₄ areapproximately the same, from the above equation (1) a ratio of the twosignals S₁₂(T) and S₃₄(T) corresponding to the following equation (2)can be formed from the output signals $\begin{matrix}{{S_{12/34}(T)} = {\left( {{S_{P,12}(T)}*ɛ_{12}*C_{12}} \right)/\left( {{S_{P,34}(T)}*ɛ_{34}*C_{34}} \right)}} \\{= {\left( {{S_{P,12}(T)}*C_{12}} \right)/{\left( {{S_{P,34}(T)}*C_{34}} \right).}}}\end{matrix}$

This allows an actual temperature of the object surface being consideredto be determined relatively reliably even if the band emittances aredifferent. In addition, since the two factors which depend on the opticsused in each case and the sizes of the surfaces being considered areknown, equation (2) is reduced to the following equation (3):S _(12/34)(T)=S _(P,12)(T)/S _(P,34)(Y)*C _(Optik)

In this equation (3) the factor C_(Optik) is merely contained as ageneral factor which describes the special properties of the twopyrometer channels. The calculated value S_(12/34)(T) is a relativelyreliable measure for the temperature of the surface being considered.The invention thus makes it possible to record the cooking temperatureindependently of the material. In this case, there are no problems withfluctuating enamel qualities or with different materials of the cookingutensils used since the infrared cooking sensor is independent of thetype of cooking utensil used.

If the two band emittances ε₁₂ and ε₃₄ are not the same, the followingequation (4)C _(ε)=ε₁₂/ε₃₄can then be used to determine a factor C_(ε) from a ratio of the twoband emittances. According to the above equation (3), the signal ratioS_(12/34)(T) can thus be determined nevertheless and specifically inaccordance with the following equation (5):S _(12/34)(T)=S _(P,12)(T)/S _(P,34)(T)*C _(Optik) *C _(ε)

If the accuracy achieved with these equations should not be sufficient,more than two wavelength ranges can also be considered as desired. Byconsidering at least three separate wavelength ranges λ₁≦λ≦λ₂, λ₃≦λ≦λ₄and λ₅≦λ≦λ₆ (where λ₂<λ₃ and λ₂<λ₃), the measurement accuracy can beincreased and in addition, the reliability of a calculation according toEquation (3) can be checked. Optionally, it is even possible to estimatethe ratio of the band emittances being considered. The quality of suchan estimate can give very accurate values for the temperatures to berecorded in the particular application being considered since the numberof different cooking utensils used is not unlimited. As a rule, thecooking sensor is only confronted with a very limited number ofdifferent materials for cooking utensils.

For a relatively precise temperature control, it can be advantageous ifthe individual wavelength ranges do not overlap but adjoin one anotheror are separated from one another. In this case, it can be advantageousfor a reliable prediction of the temperature if the amounts of energyallocated to the wavelength ranges are substantially the same.

For a particularly accurate calculation of the temperature, it ispreferable if the wavelength ranges have a width of at least 5 to 2 μm,in particular a width of 10 μm to 20 μm. In practice, it has been shownthat a restriction of the recorded wavelength ranges to 15 μm to 20 μmis sufficient to allow a relatively precise temperature control. In thiscase, it is advantageous for a temperature determination which is aserror-free as possible if on the one hand, the individual wavelengthranges are the same width as far as possible and at the same time, abutagainst one another.

The invention further relates to a cooking device comprising at leastone hot plate which has an electronic temperature control whereof atleast one control variable can be determined by means of a temperaturesensor according to one of the previously described embodiments. Theappropriate wavelength ranges for the multi-channel pyrometer of thetemperature sensor can lie between 4 and 20 μm. The cooking sensorpreferably comprises a multi-channel pyrometer comprising at least threemeasurement wavelength ranges. Optionally, more channels can also beprovided.

The invention finally relates to a method for non-contact recording of atemperature of a cooking utensil wherein at least two differentwavelength ranges emitted by the cooking utensil can be recorded bymeans of an infrared sensor.

The advantage of the sensor according to the invention is that no moreproblems can arise with fluctuating enamel qualities, differentmaterials, different preliminary damage to the cooking utensils, etc.The cooking sensor is relatively independent of the type of cookingutensil used.

Further embodiments and advantages of the invention can be deduced fromthe dependent claims.

The invention is explained in detail hereinafter using a preferredexemplary embodiment with reference to the appended drawings. In thefigures:

FIG. 1 is a schematic diagram of a hot plate provided with a temperaturesensor according to the invention,

FIG. 2 is a diagram showing a relationship between a temperature of acooking utensil and radiation energy emitted by said utensil and set

FIG. 3 is a further diagram showing different signal travel profiles ofa black body emitter at different temperatures or wavelength ranges.

FIG. 1 illustrates a hot plate 10 of a cooking device which comprises aheating device 12 underneath a glass ceramic plate 14. The heating powerof the electrical heater 12 can be adjusted by means of an electroniccontrol circuit 16. Located on the hot plate 10 is a cooking utensil 18with food 20 contained therein. An infrared sensor 22 which records atemperature at an outer wall 24 of the cooking utensil 18 in anon-contact manner is provided to record the temperature of the food 20in the cooking utensil 18 as accurately as possible.

At least two input quantities are processed in the control circuit 16.These are, firstly, a pre-selected cooking temperature T_(set)pre-selected by the user by means of an input device 26. Secondly, thecontrol circuit records a measured signal supplied by the infraredsensor 22. The infrared sensor 22 preferably comprises a multi-channelpyrometer whose electric output signals S(T) are evaluated in thecontrol circuit 16 and from which any compensation is calculated so thatthe temperatures of the food 20 can be reliably determined for differentmaterials of the cooking utensil 18.

The diagram in FIG. 2 illustrates a qualitative relationship between theoutput signals of the sensor in different wavelength ranges and atdifferent temperatures of the cooking utensil for the example of aso-called black body emitter. The relationships between the thermalenergy emitted by the cooking utensil at different wavelengths areshown. The wavelength λ in the range between 0 and 20 μm is plotted onthe abscissa. The energy (in W/cm²/μm) of the cooking utensil as ablack-body emitter is plotted on the ordinate.

It can be clearly seen that at a cooking utensil temperature of about40° C. (T_(pot)≈40° C.) no useable predictions can be made on thethermal energy emitted at different wavelengths. At an elevatedtemperature of the cooking utensil of about 250° C. (T_(pot)∓250° C.),however, a specific energy maximum occurs in the wavelength range beingconsidered between 5 μm (λ₁) and 7 μm (λ₂), from which a signal S₁₂ canbe obtained. Likewise, a useable signal S₃₄ can also be obtained in thefurther wavelength range being considered between 9 μm (λ₃) and 11 μm(λ₄), from which a probable temperature of the cooking utensil can bederived with relatively good accuracy using equations (1) to (5)presented above.

The temperature recording of the food cooking in the cooking utensil isbased on previously measured specific temperature profiles for differentmaterials, coatings, reflectances and wall thicknesses of cookingutensils typically used in practice. The cooking utensil used can beconcluded with a high probability from the specific signal profiles fromwhich the temperatures thereof can be derived from allocationspecifications stored in the control circuit.

The diagram in FIG. 3 illustrates a series of signal travel profileswhen considering a black body. These profiles can be confirmed by roughcalculations. It is clear here that a signal travel in the order ofmagnitude of 3 to 5 can be reckoned on for infrared measurements in atemperature range of about 40° C. to about 200° C. (within a wavelengthrange of about 5 μm to about 16 μm). Whereas a pot temperature (T_(pot))in a range of 0° C. to 200° C. is plotted on the abscissa in the diagramin FIG. 3, the ordinate shows the predicted signal travel SH. Whilst thehorizontal curve describes the signal S₁₂ in a wavelength rangeconsidered between 8 μm and 9 μm, the five sloping curves indicate asignal travel SH of the second signal S₃₄ being considered, whichdepends on the pot temperature, in respectively different wavelengthranges. Relatively precise conclusions regarding the material beingconsidered can be drawn from these relationships so that in reality thetemperature sensor delivers a material-dependent signal that isconverted into a material-independent temperature signal in thesubsequent evaluation unit.

The restriction of the wavelength ranges recorded to about 5 to 15 μm issufficient in practice to allow relatively precise temperature control.The simultaneous recording of three wavelength ranges significantlyincreases the quality of the control compared with recording only tworanges. Recording four or more ranges can significantly improve thequality of the temperature control still further.

REFERENCE LIST

-   10 Hot plate-   12 Heating device-   14 Glass ceramic plate-   16 Control circuit-   18 Cooking utensil-   20 Food-   22 Infrared sensor-   24 Outer wall-   26 Input device

1-11. (canceled)
 12. A temperature sensor of a cooking device fornon-contact recording of a temperature of a cooking utensil, thetemperature sensor comprising: at least one infrared sensor having atleast two different sensitivity ranges for recording at least twodifferent wavelength ranges emitted by the cooking utensil.
 13. Thetemperature sensor according to claim 12, wherein the infrared sensorcomprises a multi-channel pyrometer.
 14. The temperature sensoraccording to claim 12, wherein the wavelength ranges are a selected oneof a group of wavelength ranges that adjoin one another and a group ofwavelength ranges that are spaced apart from one another.
 15. Thetemperature sensor according to claim 12, wherein each of the wavelengthranges is allocated substantially the same amounts of thermal energy.16. A cooking device comprising: at least one hot plate having anelectronic temperature control with at least one control variable thatcan be determined by means of a temperature sensor (a) operable torecord in a non-contact manner a temperature of a cooking utensildeployed in connection with cooking on the hot plate and (b) having atleast one infrared sensor with at least two different sensitivity rangesfor recording at least two different wavelength ranges emitted by thecooking utensil.
 17. The cooking device according to claim 16, whereinthe electronic temperature control has at least one control variablethat can be determined by means of a temperature sensor arranged on thehotplate in the immediate vicinity of the cooking utensil.
 18. A methodfor non-contact recording of a temperature of a cooking utensilcomprising: recording at least two different wavelength ranges emittedby the cooking utensil by means of an infrared sensor comprising amulti-channel pyrometer.
 19. The method according to claim 18, whereinrecording at least two different wavelength ranges emitted by thecooking utensil by means of an infrared sensor includes recordingwavelength ranges lying in a range between about 4 μm to 20 μm.
 20. Themethod according to claim 18, wherein recording at least two differentwavelength ranges emitted by the cooking utensil by means of an infraredsensor includes recording at least three different wavelength ranges.21. The method according to claim 18 and further comprising determiningthe temperature of the cooking utensil from signals respectivelyassociated with the different wavelength ranges.
 22. The methodaccording to claim 18 and further comprising taking into accountdifferent band emittances of different materials of the cooking utensilwhen determining the temperatures.